1. Field of the Invention
The present invention relates generally to a linear displacement transducer with improved accuracy and, more particularly, to a linear displacement transducer with improved accuracy, in which stable contact is achieved by forming contacts coming into contact with resistors in a elastic linear contact type, rather than a conventional point contact type, using a rotating roller made of a conductive rubber material, the life span thereof is extended by considerably reducing friction, and the accuracy thereof is improved.
2. Description of the Related Art
Generally, linear displacement transducers, which are generically called “potentiometers,” employ a principle in which an electrical resistance value varies according to displacement when a contact moves on a linear electric resistor. As shown in FIG. 1, when it is assumed that the electrical resistance value of a linear electric resistor 10 is RL, the electrical resistance value RD between contacts B and C is expressed by Equation 1,                               R          D                =                              D            L                    ⁢                      R            L                                              (        1        )            where L is the total length of the linear electric resistor 10 and D is the distance between the contacts B and C. The principle imports that, if it is assumed that the electric resistor 10 has a uniform resistance value, that is, the electric resistor 10 is linearly formed, displacement (distance) is proportional to electrical resistance.
When Equation 1 is rearranged with respect to D Equation 1 is expressed as shown in Equation 2,                     D        =                              L                          R              L                                ⁢                      R            D                                              (        2        )            
Since L and RL are constant values, it is concluded that the value of D, that is, displacement, can be obtained by measuring RD. The linear displacement transducers employing such a principle are mounted on and widely used in precision machining devices, such as a lathe and a catapult, that need the measurement of displacement.
Generally, one of the most frequently used linear displacement transducers is the LTM model from Italian GEFRAN company, which has an appearance with a square cross section (not shown). The measurement length of this measuring instrument is determined by the length of the appearance (chassis) (50 to 900 mm), and a moving shaft 20 having a circular cross section, which forms contacts in the interior of the measuring instrument, changes electric resistance while moving inward and outward. That is, the measuring instrument measures the displacement of a corresponding object in such a way that one end of the moving shaft 20, which moves along the interior of the appearance having a square cross section, is secured on the object whose displacement needs to be measured as shown in FIG. 2, and the variation of electrical resistance attributable to the movement of the object is measured.
In the interior of the appearance along which the moving shaft 20 moves, linear resistance plates, which are manufactured in the shape of a Printed Circuit Board (PCB) 30 and formed to come into contact with the moving shaft 20, are secured. As shown in FIG. 2, the moving shaft 20 is constructed to move while being in contact with a thin film A, which is manufactured in the shape of a PCB 30 and made of a pure conductive material having zero electrical resistance, and a thin film B, which is manufactured in the shape of a PCB 30 and made of a resistance material having electrical resistance RL, through a connection line 40 that is also made of a pure conductive material (electrical resistance=0).
Accordingly, when the electrical resistance between contacts D and E, which are formed by the connection line 40 while the moving shaft 20 moves, is measured, the value of RD of Equation 2 can be measured because the thin film A is a pure conductor having zero electrical resistance. That is, since the connection line 40 secured on one end of the moving shaft 20 moves as the moving shaft 20 moves, the value of RD that is proportional to the displacement of the moving shaft 20 can be obtained. In the case of the LTM-550 model that can measure displacement in a range of 550 mm, a linear relationship Equation as shown in FIG. 3 can be obtained at the time of measuring the value of RD while moving the moving shaft 20 at several mm intervals, and the value of D, that is, displacement, can be obtained from the measured value RD using the linear relationship Equation.
However, since the conventional linear displacement transducer is basically constructed to form the point contacts between the connection line 40 and the thin film A and between the connection line 40 and the thin film B, the conventional linear displacement transducer is problematic in that accurate displacement cannot be measured in the case where the moving shaft 20 and the connection line 40 connected to the moving shaft 20 are rotated (or twisted).
That is, since the thin film B forming point contact with the connection line 40 is not a pure conductor but a linear electric resistor formed in a linear plate shape, only an electrical resistance value is measured on a single point of the connection line 40 in contact with the thin film B. Accordingly, in the case where the moving shaft 20 moves while the moving shaft 20 is not completely parallel to the thin film B, the moving shaft 20 is rotated (or twisted) as shown in FIG. 4, so that a problem arises in that an electrical resistance value measured on the thin film B is changed because the location of the contact of the thin film B in contact with the connection line 40 is changed, whereas the location of the contact of the thin film A is not considered important because the thin film A is a pure conductor.
In practice, at the time of measurement of a linear displacement measuring instrument, there is almost no case where the moving shaft 20 moves while remaining completely parallel to the thin films A and B, and the connection line 40 connected to the moving shaft 20 moves while remaining completely perpendicular to the thin films A and B, so that the accuracy of displacement measurement is determined according to the extent of the rotation (or twist) of the moving shaft 20. Some rotation phenomenon must be allowed in order to allow a moving shaft to freely move forward and backward, so that a problem arises in that measurement accuracy is reduced. Furthermore, the resistance material of the thin film B cannot be completely uniform, so that a problem arises in that a resistance value varies according to the locations of point contacts even when the displacement is the same.
Furthermore, elastic contacts are formed between the connection line 40 and the thin film A and between the connection line 40 and the thin film B by applying predetermined pressure from the above in order to allow the connection line 40 secured on the moving shaft 20 to form electrically stable contacts, so that an unavoidable wear phenomenon occurs due to the movement of the moving shaft 20, thus reducing the life span of the measuring instrument.
The wear phenomenon is caused by friction between the two secured solid bodies. Contacts are unstable and contact noise occurs if contact pressure is reduced to reduce the friction, thus causing the reduction of measurement accuracy. In practice, when resistance values are measured while the moving shaft 20 of an LTM-550 product moves at 0.1 mm intervals, it is concluded that linearity is considerably reduced as shown in FIG. 5 (correlation coefficient is 0.8596). The reason for this is that the amount of the forward and backward movement of the moving shaft 20 is considerably smaller than the amount of the rotation of the moving shaft 20. When the accuracy of the LTM-550 product is repeatedly measured five times and an average value of correlation coefficients is calculated, a correlation coefficient is 0.9987 when the amount of minimal displacement is 1 mm, and a correlation coefficient is 0.8948 when the amount of minimal displacement is 0.1 mm.